Electronic devices of recent years have been increasingly miniaturized, reduced in weight and advanced in functionality. Against this backdrop, the trend toward multi-layered, high-density printed circuit boards (hereinafter, PCB) has been gathering its momentum. A PCB is typically produced by alternately stacking a plurality of substrates with conductive circuits formed thereon and prepreg sheets (so-called “prepreg”), bonding them under heat and pressure, forming through-holes, and plating them with copper or other metallic materials to provide an electrical connection between the surface and inner layers.
Due to an intensified demand for video cameras and mobile telecommunication devices, however, PCBs used for such devices have been ever more strongly required to be lighter, thinner and high density. In response to such demand, a manufacturing method for a new kind of PCB disclosed in Japanese Patent No. 2601128 has been put into practical application. The manufacturing method of this PCB comprises the steps of:                1. forming through-holes on a porous compressible substrate sandwiched between releasing films;        2. filling the through-holes with conductive paste;        3. removing the releasing films;        4. placing metal foils on both sides of the porous substrate and compress the laminate under heat to provide an electrical connection; and        5. etching the metal foils to form circuitry.        
The configuration of the foregoing conventional PCB is described with reference to the attached drawings. FIGS. 8(a)-(g) are sectional view showing the manufacturing process of the conventional PCB. As FIG. 8(a) shows, a porous substrate (hereinafter, prepreg) 11 whose dimensions are 500 mm square and T1 mm in thickness, sandwiched between releasing films 12, is prepared. As the prepreg 11, a composite material produced by impregnating thermosetting epoxy resin into non-woven fabric made of aromatic polyamide fiber is used.
Successively, as FIG. 8(b) shows, through-holes 13 are formed at the predetermined places of the prepreg 11 by a laser machining method.
The prepreg 11 is then placed on a table of a printer (not illustrated), and conductive paste 14 is printed on the releasing films 12 to fill the through-holes 13 as FIG. 8(c) shows. In this process, the releasing film 12 coating the top face of the prepreg 11, acts as a printing mask and an anti-contamination material for the prepreg 11.
Successively, as shown in FIG. 8(d), the releasing films 12 coating both faces of the prepreg 11 are removed at room temperature. Metal foils 15 made of copper or other metals are attached to both faces of the prepreg 11 as shown in FIG. 8(e). The laminate is then pressed under heat to adhere the prepreg 11 and the metal foils 15 as illustrated in FIG. 8(f). Simultaneously, in this process the prepreg 11 is compressed to T2 mm in thickness (T1>T2) to electrically connect the metal foils 15 on both of its faces with the conductive paste 14.
During this step, the epoxy resin composing the prepreg 11 and the conductive paste 14 are cured. In order to lower connection resistance between the conductive paste 14 and the metal foils 15, as disclosed in the U.S. Pat. No. 2,587,596, after filling the conductive paste 14 and removing the releasing films 12, the prepreg 11 is provided with protrusions of the conductive paste on both of its faces (see FIG. 8(d)).
If the conductive paste 14 protrudes from the top and bottom faces of the prepreg 11 as disclosed in this method, the protrusions allow the conductive paste 14 to be compressed more and become denser than a method without protrusion when the metal foil 15 and the prepreg 11 are pressed under heat. Therefore each of metal powders contained in the conductive paste 14 contact in larger area with each other and with the metal foils 15 placed on both front and back faces of the prepreg 11. This results in lower resistance in the connection.
Successively, as shown in FIG. 8(g), after the metal foils 15 are provided with patterns by photo lithography, both of the metal foils 15 on cured faces of the prepreg 11 are etched to form wiring patterns 16.
According to the conventional configuration, however, when compressed, the conductive paste protruding from the prepreg fails to be kept in the through-holes and overflows from them, and connects to other wiring patterns to trigger a short circuit.
The present invention aims to address the foregoing problems, and providing a PCB which achieves a reliable electrical connection between the conductive paste and the metal foils disposed on both faces of the prepreg. The PCB of the present invention allows the conductive paste to be filled into the through-holes properly, and even when the wiring patterns has high density, avoids short circuits triggered by connection between the conductive paste and an undesignated wiring pattern.